This invention relates to a CATV system in which the central facility (hereinafter referred to as "a center", when applicable) is connected through cables to a number of terminal units so that communication is effected between the center and the terminal units, and more particularly to an unjust, or unauthorized, observation preventing device for a CATV system which can prevent programs from being unjustly watched at the terminal unit.
A CATV system in which the invention may be employed will be described with reference to FIG. 1. The CATV system includes a single base or center 1 and terminal units 28 which are provided, for example, in several tens of thousands of subscriber's homes. The center 1 is connected through coaxial cables to the terminal units 28. A main cable 3 extends from the center 1 and main cable amplifier 4 and branching units 5 are provided at predetermined positions on the main cable 3. A plurality of branch cables 6 extend from each branching unit 5. Extension amplifiers 7 and tap-off units 8 are provided at predetermined points on the branch cables 6. Each tap-off unit 8 is connected to branch lines 9 which extend to the terminal units 28 in individual homes 2. Each terminal unit 28 includes a main unit 10, a television set 11 and a control unit 12. The branch line 9 is connected to the main unit 10 which is in turn connected to the television set 11 and to the control unit 12. As is apparent from the above description, the main cable extending from the center is branched into a first plurality of branch cables which are further branched into a second plurality of branch lines which are finally connected to the terminal units 28.
A signal receiving antenna 13 is installed outside the center 1. The antenna 13 is connected to a demodulator 15 in a source group 14 of the center 1. The source group 14 further includes a video disc player 16, a video tape recorder 17 and a studio 18. The output signals of the source group 14 are applied to a modulation output section 19 composed of two systems. One of the two systems includes an IF modulator circuit 20, a scramble circuit 21, and an up-converter circuit 22, while the other system includes an IF modulator circuit 23 and an up-converter circuit 24. The outputs of the up-converters 22 and 24 are connected to the main cable 3, to which a data transceiver 25 is connected for data communication between the center 1 and each main unit 10. The data transceiver 25 is connected to a computer 26 which is connected to a peripheral unit 27 such as a printer or a display unit.
The operation of the CATV system thus constructed will now be described.
First, the power switch of the television set 11 is turned on. With the television set 11 set to a particular unused channel, the control unit 12 is operated so that the frequency of a desired channel to be received is set to that of the unused channel. The channels which can be selected by the control unit 12 as described above can be classified into three groups of channels for (A) retransmission programs in which television signals from local stations are received without modification, (B) independent programs (free of charge), and (C) chargeable (pay) programs. Each group is allocated ten channels, and therefore any of the thirty channels can be selected by operating the control unit 12.
(A) Retransmission programs
Television signals received by the antenna 13 are demodulated by the demodulator 15 and applied to the modulation output section 19. The signal thus applied is modulated by the IF modulator circuit 23. The frequency of the signal thus modulated is increased to a predetermined frequency by the up-converter circuit 23 according to a determined frequency allocation scheme. That is, it is assigned to a predetermined channel. The resultant signal is applied through the main cable 3, the branch cables 6, and the branch lines 9 to the television sets 11.
(B) Independent programs
The independent programs include locally originating programs such as weather forecast programs, news programs, and the like. In the case of programs recorded on discs of the video disc player 16 or on the video tape recorder 17 or in the case of live programs produced in the studio 18, the video signal is modulated by the IF modulator circuit 23 and the frequency of the signal thus modulated is increased to that of a predetermined channel by the up-converter circuit 24. The resultant signal is applied to the main cable 3. Reception of these programs is free of charge. That is, the "price" of these programs is included in the basic monthly charge paid by the subscriber.
(C) Chargeable programs
The chargeable programs include new movie programs, special programs, and the like. In the case of programs provided by the video disc player 16 or the video tape recorder 17 or in the case of live programs produced in the studio 18, the video signal is modulated by the IF modulator circuit and is then processed by the scrambler circuit 21 so that the signal cannot be reproduced without special signal processing. Then, the frequency of the signal is increased to that of a predetermined designated channel, and is applied to the main cable 3. In order to receive the chargeable programs, the video signal must be descrambled by the main unit 10 in the subscriber's home 2 so that a regular image appears on the television set 11. The subscriber is charged for the reception of the chargeable programs. That is, predetermined charges therefore are summed, and the user is requested to pay a special charge at a measured rate in addition to the basic monthly charge.
As described above, the center 1 is connected through coaxial cables to the terminal units in the subscribers' homes 2. However, in order to charge the subscribers for the reception of the chargeable programs, it is absolutely necessary to detect which subscribers receive which channels. That is, it is necessary to detect the channels which have been selected by the subscribers at various times. In order to meet this requirement, the data transceiver 25 outputs a retrieving signal at predetermined time intervals so that the terminal units 28 are called via their assigned address numbers. In this way, the channels received by the terminal units at the time of transmission of the retrieving signal are detected, this operation being referred to as "polling." In response to this polling, each terminal unit 28 "answers" the data transceiver 25 with data representative of the channel received by the terminal unit at the time of polling. Various reception and transmission data for the data transceiver 25 are arranged and stored by the computer 26 and are displayed or printed out by the peripheral unit 27. The polling operation is carried out at predetermined time intervals of several seconds to several tens of seconds and therefore audience ratings can be readily calculated.
Sometimes, the subscribers may participate in the production of programs. In this case, by operating the control units 12, they can answer questions proposed in a program or the like while watching the television sets 11. The answers are transmitted through the coaxial cables to the center 1.
The bi-directional data communication between the center and the terminal units 28 is as described above. In order to satisfactorily run the system, it is necessary for the center 1 to detect (or know) the operating conditions of the terminal units 28 at all times. For instance if the center cannot prevent chargeable programs from being watched unjustly, then it is impossible to correctly and fairly charge the subscribers, and finally it is difficult to smoothly run the system. Thus, it is essential for the CATV system to prevent the unauthorized observation of programs. Heretofore, for this purpose, the center 1 has transmitted the video signal and the data signal "downstream" to the terminal units 28 at all times, while the terminal units 28 send "upstream" data signals to the center 1 to indicate what channels they are using so that the center can detect the operating conditions of the terminal units 28, as described above. However, if some means is used to interrupt the reception of an operating condition retrieving signal included in the "downstream" data signal, or to obstruct the transmission of the "upstream" data signal by which the channel using conditions are informed to the center 1, then the center cannot detect the operating conditions of the terminal unit 28 any longer, and cannot prevent the unauthorized observation of programs.